Airlines often retain engineers at there maitainance bases in several different capacities. Most I notice are there to act as a sort of liason between the manufacturer and the airline as they research and generate specific items such as structural repairs and modifications made in the field, ie: Some may be designing a skin repair, some designing a retrofit of an avionics system, IFE, some may just design moving the spare bulb kit from the side-wall in the cockpit to the rear wall of the cockpit, or beefing up of cockpit doors in the wake of the 9/11 attacks. Just isolated examples. Many are not A&P's. -- The travelling/troubleshooting that you mention is somewhat different- although there are some techincal representatives ( or field service technicians, or wahtever they're called at a specific airline ) who are engineers, most who fill that job were A&P mechanics with many years of experience on a variety of A/C types owned by that specific airline. -- You will also find various representitives and engineers for the A/C or specific system thereof from the A/C manufacturer working at an airline. -- This is what I see from 18 years in the business, but I'm sure an engineer so employed can explain much better than I.